In recent years, keen demand has arisen for reduction of CO2 emission in order to prevent global warming. One of the most important measures for reduction of CO2 emission is to improve fuel efficiency of automobiles; i.e., fuel-saving performance.
Saving of automobile fuel can be attained through reduction of the bodyweight of vehicles, improvement of combustion mechanism; e.g., employment of combustion of lean mixture, and improvement of fuel-saving performance of engine oil (lubricating oil for internal combustion engine).
The fuel-saving performance of engine oil is considered to be improved essentially through the following techniques: reducing the viscosity of engine oil so as to reduce friction loss, which is caused by lubricating oil fluid present in a fluid-lubrication area, and reducing friction generated by engine oil in order to reduce friction of sliding parts present in a mixed lubrication area.
However, when the viscosity of an engine oil is reduced excessively in an attempt to reduce friction loss caused by lubricating oil fluid, oil consumption unavoidably increases, and oil film strength decreases, resulting in a drop in wear resistance. One possible technique for reducing friction of sliding parts is incorporation of a friction reducer into engine oil. However, mere addition of a large amount of friction reducer results in insufficient friction reduction effect, or failure to maintain the reduction effect for a long period of time. Thus, at present, fuel-saving performance cannot readily be attained. In order to solve this problem, a variety of studies are underway on improvement of fuel-saving performance engine oil (see, for example, Patent Documents 1 and 2).
Patent Documents 1 and 2 disclose engine oils containing additives such as Ca salicylate, an organic molybdenum-based friction reducer, and a phenol-based antioxidant. However, the friction-reducing effect of the proposed engine oils cannot be maintained for a satisfactorily long period of time, and further improvement has been needed.
Therefore, there is demand for the development of an engine oil which exhibits excellent fuel-saving performance for a longer period of time.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 5-163497
Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2002-371292